It is very important for construction machines such as hydraulic shovels and bulldozers to accurately estimate a timing of engine overhaul, that is, the operation life of the engine in order to carry out engine maintenance and inspection in accordance with a set schedule. However, there are difficulties in individually estimating the engine life of vehicles, because the engine life of construction machines varies significantly depending on their working situations and operating conditions even if they are of the same type and have the same type engines.
One attempt was previously made in which data on the load condition of an engine are collected over a long period of time and engine life is estimated from changes with time in the load condition. This method is effective where the engine is continuously operated under a constant load, but it has encountered considerable difficulties when applied to cases where engine load changes with time.
The present applicant has proposed one solution to the above problem in a previous invention (entitled "A device for estimating the life of a machine having an engine and heat source", Japanese Patent Publication (KOKAI) Gazette No. 11-211622 (1999)). In this prior invention, the values of operation parameters indicative of the load imposed on an engine are classified into a plurality of levels thereby setting a load map; the time of detecting a parameter value belonging to each level of the load map is accumulated; the actual amount of damage to the engine is calculated based on the accumulated time; and the engine life corresponding to the calculated amount of damage is obtained from preset data indicative of the relationship between the degree of damage and the length of life. As the operation parameters mentioned herein, the rotational speed of an engine, output torque (or the amount of fuel injection) and the like are selected.
In the previous invention, while detection data from an engine rotational speed sensor mounted for controlling basic performance can be utilized in the detection of the rotational speed of the engine, a specialized sensor such as a rack position sensor or pump swash plate sensor needs to be newly adapted for the detection of output torque. This results in increased cost.
On the other hand, there is a technique for estimating output torque without addition of a specialized sensor. Concretely, a nominal line in an engine output characteristic graph such as shown in FIG. 9(a) is stored in the controller beforehand, and an output torque T' is estimated from an actual rotational speed Na detected by an engine rotational speed sensor. This estimation technique however has revealed a difficulty in ensuring detection accuracy because there occurs reversal in the relationship between detected load and actual load such as shown in FIG. 9(b). More specifically explaining with reference to FIG. 9(b), when comparing the two engine data represented by chain lines A and B which vary in regulation relative to the ideal line represented by solid line, the relationship between actual torque T(A) and actual torque T(B) relative to engine rotational speeds Na(A) and Na(B) is represented by T(A)&gt;T(B), whereas the relationship between estimated torque T'(A) and estimated torque T'(B) is represented by T'(A)&lt;T'(B). As understood from this, the actual torque relationship is opposite to the estimated torque relationship.
The invention has been directed to overcoming the above problems and a primary object of the invention is therefore to provide a data processing unit for a construction machine, the unit being capable of storing data on load condition independently of variations in engine output torque and performing data processing such as estimation of the operation life of an engine based on the stored data, without additionally incorporating a sensor which entails increased cost.